1. Field of the Invention
The present invention relates to an absorbent article capable of absorbing and retaining body exudates such as typically menses, in particular, to such an absorbent article having a topsheet that rapidly passes a large amount of liquid therethrough so as to keep its surface still in a dry condition while absorbing a small amount of liquid but not having a wet feel.
2. Description of the Related Art
Heretofore, various types of absorbent articles such as sanitary napkins, urine pads and disposable diapers have been developed. These absorbent articles comprise a liquid-impermeable back layer (backsheet), a liquid-permeable top layer (topsheet) and an absorbent layer (absorbent core) positioned between the two sheets.
Regarding its structure, for example, the liquid-permeable top layer comprises a thermal-bonded nonwoven fabric of hydrophobic fibers having a relatively large porosity, or comprises a nonwoven fabric mainly of cellulosic fibers in its entirety as in Japanese Patent Laid-Open No. 24289/1996. In addition, Japanese Patent Laid-Open No. 10209/1979 discloses a top layer that comprises a nonwoven fabric of from 70 to 85% by weight of hydrophobic synthetic fibers and from 15 to 30% by weight of hydrophilic fibers.
When the aforementioned absorbent article that comprises a top layer of hydrophobic fibers has received a large amount of liquid, the liquid applied to its top layer is immediately brought into contact with an absorbent layer positioned therebelow, and the liquid within the top layer is drawn by the absorbent layer owing to the hydrophilic capillary action of the absorbent layer. In this stage, since the fibers constituting the top layer is poorly resistant to the liquid penetration (movement) through them, almost all the liquid having been applied to the top layer is rapidly drawn by the absorbent layer, not remaining in the top layer, and the surface of the top layer is therefore readily kept in a dry condition.
However, when the top layer has received a small amount of liquid or the sweat of a wearer, such a small amount of liquid will adhere to the fibers constituting the top layer but is hardly brought into contact with the absorbent layer. In that condition, therefore, the drawing force of the absorbent layer does not act on the small amount of liquid, and, as a result, the small amount of liquid often remains in the space between the hydrophobic fibers constituting the top layer. In addition, since the hydrophilic capillary action of the top layer of hydrophobic fibers is poor, the small amount of liquid remaining in the top layer often returns to the skin of a wearer. Accordingly, the top layer is liable to give a wet feel to a wearer and cause skin roughness.
In order to impart hydrophilic nature, it is also known in the art to coat the surfaces of the hydrophobic fibers with a surfactant or the like. However, when a large amount of liquid is applied, it washes the surfactant or the like away from the surfaces of the hydrophobic fibers to lower the hydrophilic nature of the fibers. Accordingly, a small amount of liquid remaining in the space between the hydrophobic fibers hardly moves toward the absorbent layer, and it still remains in that space.
On the other hand, the top layer mainly of cellulosic fibers in its entirety, as in Japanese Patent Laid-Open No. 24289/1996, gives a soft feel to the skin of a wearer, as its material is soft per se. When it has received a small amount of liquid or the sweat of a wearer, in addition, the cellulosic fibers retain the liquid, and, moreover, the small amount of liquid is well retained within the top layer owing to the hydrophilic capillary action of the fibers. Therefore, the liquid hardly returns to the skin of a wearer.
However, since the top layer of the type exhibits hydrophilic capillary action by itself, when a large amount of liquid is applied to the top layer, it does not rapidly move toward the absorbent layer, and it remains within the top layer. This results in a wet feel to the skin of a wearer.
In the top layer of a nonwoven fabric formed by mixing hydrophobic fibers and hydrophilic fibers as in Japanese Patent Laid-Open No. 10209/1979, the hydrophilic fibers are uniformly dispersed among the hydrophobic fibers. Accordingly, when a large amount of liquid is applied to the top layer and while it passes through the space between the hydrophobic fibers therein to move toward the absorbent layer, a part of the liquid is retained by the hydrophilic fibers and will therefore remain in the top layer. For this reason, the permeation rate of a large amount of liquid through the top layer is low.
An object of the present invention is to provide an absorbent article having a topsheet capable of smoothly passing a large amount of liquid therethrough and retaining a small amount of liquid therein so that it hardly gives a wet feel to the skin of a wearer to thereby improve a wear feel.
According to an aspect of the invention, there is provided an absorbent article comprising a topsheet, a backsheet and an absorbent core positioned between the topsheet and the backsheet, wherein;
the topsheet contains hydrophobic fibers and hydrophilic fibers shorter than the hydrophobic fibers, the hydrophobic fibers are thermally bonded to each other, at least a part of the hydrophilic fibers form aggregates that disperse in the sheet, and at least a part of the hydrophilic fibers that form the aggregates are bonded to the surfaces of the hydrophobic fibers.
In the above, preferably, the fiber density of the hydrophilic fibers that form the aggregates is higher than the fiber density of the area of the sheet not having the aggregates therein. More preferably, the fiber density of the hydrophilic fibers that form the aggregates falls between 1.5 times and 3 times the fiber density of the area of the sheet not having the aggregates therein.
When the topsheet is divided into two layers in the direction parallel to its thickness, one being a face layer that receives liquid and the other being a back layer adjacent to the absorbent core, it is desirable that the hydrophilic fiber aggregates are not in the face layer but are only in the back layer. Thus constituted, the hydrophilic fiber aggregates do not drop away from the face layer of the topsheet that receives liquid, and the surface strength of the liquid-receiving face layer of the topsheet can be high.
In the above, preferably, the basis weight of the topsheet falls between 20 and 60 g/m2, and the basis weight of the face layer of the topsheet falls between 5 and 15 g/m2. Thus constituted, the distance between the liquid-receiving surface of the face layer of the topsheet and the hydrophilic fiber aggregates existing in the back layer of the topsheet can be shortened, so that the liquid applied to the face layer of the topsheet can be readily absorbed by the hydrophilic fiber aggregates.
For example, the hydrophobic fibers have a fiber length of from 38 to 64 mm; and the hydrophilic fibers have a fiber length of from 5 to 25 mm. Preferably, the fiber length of the hydrophilic fibers is at most xc2xd of the fiber length of the hydrophobic fibers. If the fiber lengths are within the ranges, the hydrophilic fibers can be readily formed into the aggregates and dispersed among the hydrophobic fibers by using a conventional carding unit.
Preferably, the amount of the hydrophobic fibers in the topsheet falls between 70 and 98% by weight, and that of the hydrophilic fibers therein falls between 2 and 30% by weight. If the amount of the hydrophilic fibers in the topsheet is 30% by weight or less, more preferably 10% by weight or less, the hydrophilic fiber aggregates can be well dispersed in the sheet while suitably spaced from each other.
Preferably, the hydrophilic fibers are natural cellulose fibers having a modified cross section or a hollow cross section.
According to the invention, the topsheet is mainly formed by thermally bonding the hydrophobic fibers and the hydrophilic fibers aggregates having a higher density are dispersed therein. Therefore, when a large amount of liquid is applied to the topsheet, the liquid is drawn by the hydrophilic capillary action of the absorbent core and rapidly passes through the area of the sheet consisting essentially of hydrophobic fibers to be absorbed by the absorbent core.
When a small amount of liquid or the sweat of a wearer is applied to the topsheet, on the other hand, it is readily retained by the high-density, hydrophilic fiber aggregates dispersed in the topsheet owing to their hydrophilic capillary action. Accordingly, the small amount of liquid remains little in the space between the hydrophobic fibers in the topsheet, and returns little to the skin of the wearer. Therefore, the surface of the topsheet is well kept in a dry condition, and a wet feel is hardly given to the skin of a wearer.